The invention relates to a hydraulic power control device especially for a vehicle powered braking system, having yielding lever means. The lever means comprise an arm pivotally mounted on a first axle secured to the device housing, and a feeler member pivotally mounted on a second axle, said second axle being pivotally mounted on the arm, the ends of the feeler member cooperating with spool valve and a servomotor piston respectively, the connection between the arm and the housing being yieldable in case of excessive stresses applied on a push rod engaging the free end of the arm. A brake booster of this kind is known from the disclosure of Carre U.S. Pat. No. 3,805,671.
In this known brake booster, the slide sheath is disposed firmly in the housing. This has the disadvantage that the control valve is not trackable by simple means. Instead, a special spring lever is required in order to attain this tracking. A spring lever of this kind, however, is quite delicate and vulnerable to failure.